The present invention relates to a film thickness measuring method for detecting a film thickness of a member to be processed with use of an emission spectroscope in such processes as fabrication of semiconductor integrated circuits and a processing method of the member with use of the film thickness measuring method. More particularly, the present invention relates to a film thickness measuring method of members to be processed, preferred so as to measure a film thickness of each layer formed on a substrate in etching processing that employs plasma discharge and obtain a predetermined thickness. The present invention also relates to a processing method of those members with use of the film thickness measuring method.
Dry-etching is one of the main techniques having been employed widely in fabrication processes of semiconductor wafers so as to remove layers formed with various materials thereon. Especially, the dry-etching has been employed to remove dielectric material layers or form patterns on those layers. And, the most important point for controlling process parameters is considered to be decision for endpoints of etching processing so as to stop the etching at each predetermined thickness during the processing.
The light emission intensity of a specific wavelength changes with the progress of the dry-etching processing of semiconductor wafers. One of the conventional etching endpoint detecting methods having been employed for semiconductor wafers, therefore, detects changes of such the light emission intensity of a specific wavelength from plasma during dry-etching processing so as to detect an etching endpoint of a specific film according to this detected emission intensity change. At this time, it is strongly demanded to prevent misdetection of such the endpoint of etching processing, to be caused by irregularity of the detected waveform due to a noise. A well-known method for detecting such the changes of the light emission intensity accurately is disclosed in JP-A-61-53728 and JP-A-63-200533, etc. The moving average method is employed JP-A-61-53728 and the primary least square approximation processing is performed for noise reduction in JP-A-63-200533.
Now that sizes of semiconductors are becoming smaller and the packing density of them is becoming higher, the open area ratio, (area to be etched on a semiconductor wafer) is becoming smaller. And accordingly, the emission intensity of a specific wavelength to be fetched into a light detector from a photo sensor is becoming weaker. As a result, the level of the sampling signal output from the light detector is becoming lower, so that it is becoming difficult for an endpoint determining device to detect endpoints of etching processing accurately according to such the sampling signal output from the light detector.
To detect an endpoint of etching processing so as to stop the etching, it is important that the residual thickness of a dielectric layer should actually become equal to a predetermined value. In the conventional processing, however, all the processes are monitored by a time thickness controlling technique that premises that the etching speed is fixed for all types of layers. An etching speed, for example, is found by processing sample wafers beforehand. According to this method that employs a time monitoring method, therefore, the etching processing stops when a time corresponding to a predetermined etching film thickness is up.
However, an actual film, for example, an SiO2 layer formed by the LPCVD (Low Pressure Chemical Vapor Deposition) method is well known as a layer that is low in reproducibility. The allowable error of film thickness to occur due to a processing fluctuation in the LPCVD is equivalent almost to 10% of the initial thickness of the SiO2 layer. Consequently, the time monitoring method cannot measure the actual final thickness of the SiO2 layer left on the subject silicon substrate. And, final measurement of the actual film thickness is done with use of a standard spectroscopic interferometer. When over-etching is detected, the subject wafer is discarded as an NG one.
It is also well known that an insulation film etching apparatus often causes etching speed-down with time while the etching is repeated. Sometimes, the etching stops on the way. Such the problem must be avoided. In addition, it will also be important to monitor changes of the etching speed with time so as to assure stable etching processing. And, none of the conventional methods has been effective to cope with such the changes and fluctuations of the etching speed with time; the method just monitors the time for determining the end of etching processing. Besides, the decision for the end of etching processing has not been satisfactory when the etching time is as short as about 10 seconds, since the preparing time for the decision, as well as the decision time unit must be as short as possible. Furthermore, an insulation film area to be etched is often less than 1%, so the change of the plasma light emission intensity from a reaction product generated by etching is so small. This is why there has not been practical and reasonable price systems so far, although an etching endpoint decision system that can detect even a slight change of a light emission intensity has been demanded.
On the other hand, there are other well-known methods for detecting endpoints of etching processing on semiconductor wafers. The methods are disclosed in JP-A-5-179467, JP-A-8-274082, JP-A-2000-97648, and JP-A-2000-106356, etc. and each of those methods uses an interferometer. According to those methods that use an interferometer respectively, a monochrome laser beam is exposed at a vertical incidental angle on wafers composed of laminated layers formed with various types of materials. For example, for a wafer consisting of an SiO2 layer and an SiO3N4 layer laminated thereon, interference fringes appear on the wafer due to a light reflected from the top surface of the SiO2 layer and another light reflected from the boundary face between the SiO2 layer and the Si3N4 layer. And, the reflected lights are led into a proper detector, thereby generating a signal whose intensity changes according to the thickness of the SiO2 layer during etching processing. When the top surface of the SiO2 layer is exposed during the etching, both of the etching speed and the etched film thickness can be monitored accurately and continuously. Instead of the laser beam, a predetermined light discharged by plasma may be measured with use of a spectrometer. This is also a well-know method.
According to such a method that uses an interferometer, the position of a boundary face between laminated layers can be measured accurately. However, appearance of interference fringes due to a light reflected from the top surface of a layer and another light reflected from a boundary face means that the processing has reached the boundary face. Measurement of the position of the boundary face cannot be done before that. In actual etching processing, therefore, over-etching cannot be avoided for the target layer even when the thickness of the target film is measured online according to the interference fringes caused by those reflected lights and the information that the processing has reached the boundary face is fed back to the process control. To avoid such over-etching, therefore, the time monitoring method described above should be employed together, although the film thickness and other items must be preset in that case. And, it is becoming difficult more to do proper etching for the reasons described above under the circumstances in recent years, since higher integration of semiconductors is demanded.
Each of the conventional methods disclosed in the above gazettes will be summarized as follows.
JP-A-5-179467 discloses a method that three color filters (red, green, and blue) are used to detect an interference light (plasma light), thereby detecting endpoints of etching processing.
On the other hand, JP-A-8-274082 (corresponding to U.S. Pat. No. 5,658,418) discloses a method that changes of the interference waveforms of two wavelengths with time and their differential waveforms are used to count the extreme values (maximum and minimum values of each waveform: zero-cross points of each differential waveform) of the interference waveforms. Then, the time until the count reaches a predetermined value is measured, thereby obtaining an etching speed. And, the remaining etching time required until a predetermined film thickness is reached is measured according to the obtained etching speed, thereby stopping the etching processing according to the measured remaining etching time.
JP-A-2000-97648 discloses a method that obtains a difference waveform (that uses a wavelength as a parameter) between a light intensity pattern (that uses a wavelength as a parameter) of an interference light before processing and a light intensity pattern of the interference light after or during processing and comparing the obtained waveform with the difference waveform read from the data base, thereby measuring a difference in level (film thickness).
And, JP-A-2000-106356 discloses a rotary coating apparatus and a method for measuring a film thickness by measuring changes of an interference light with time with respect to each of multiple wavelengths.
And, U.S. Pat. No. 6,081,334 discloses a method that measures characteristic changes of an interference light with time and accumulates the measured data in a data base so as to detect an endpoint of etching processing by comparing a measured interference waveform with that read from the data base. This decision requires the etching processing conditions to be updatred.
The well-known examples described above, however, have been confronted with the following problems.
(1) As members to be etched are becoming thinner, the interference light intensity is becoming lower and the number of interference fringes to appear is reduced.
(2) When a masking material (ex., resist) is used in etching processing, an interference light from the subject member to be etched is overlaid on another interference light from the masking material.
(3) The interference waveform is warped with a change of the etching speed during the processing.
Due to the above problems, it has been difficult to measure and control the thickness of a layer to be processed, especially a layer to be processed in plasma etching processing at a required precision.
Under such circumstances, it is an object of the present invention to provide a film thickness measuring method that can measure an actual thickness of a layer to be processed online precisely in plasma processing, especially in plasma etching processing, as well as a processing method of the layer using the measuring method.
It is another object of the present invention to provide etching processing that can control each layer of a semiconductor device to a predetermined thickness online precisely.
It is still another object of the present invention to provide a film thickness measuring apparatus for a member to be processed. The method can measure an actual thickness of a layer to be processed precisely online.
In order to solve the conventional problems described above and achieve the above objects of the present invention, at first, a time differential waveform is found from an interference waveform with respect to each of a plurality of wavelengths. And, according to the found waveform, a pattern that denotes the wavelength dependence of the subject interference waveform differential value is found (that is, a pattern of a differential value of an interference waveform that uses a wavelength as a parameter). The pattern is then used to measure the thickness of a target film.
The reasons why the present invention in this specification uses a pattern denoting the wavelength dependence of a time differential value of an interference waveform are as follows:
Because film thickness measurement premises in-situ (real time) measurement during etching, the film thickness of the target film to be processed changes time to time. Consequently, time differential processing is possible for interference waveforms. Besides, this differential processing can remove noise from interference waveforms.
Furthermore, the refractivity of the member to be etched (ex., polysilicon) changes significantly with respect to a wavelength. Consequently, interference light measurement by an interferometer makes it possible to detect characteristic changes (film thickness dependence) of the member with respect to each of multiple wavelengths.
According to an aspect of the present invention, the film thickness measuring method for measuring a film thickness of a member to be processed comprises the steps of:
a) setting a standard pattern for a differential value of an interference light with respect to a predetermined film thickness of a first (sampling) member to be processed, the standard pattern using a wavelength as a parameter;
b) measuring the intensity of an interference light of a second member to be processed, composed just like the first member, with respect to each of a plurality of wavelengths, thereby obtaining a real pattern for a differential value of the measured interference light intensity, the real pattern using a wavelength as a parameter; and
c) obtaining a film thickness of the second member according to both of the standard pattern and the real pattern of the differential value.
The present invention described above may be modified as follows:
At first, in case the film of a material, which is a member to be etched, is thick, interference fringes will appear cyclically. In such a case, an absolute film thickness can be found using an interference light that has more than three wavelengths.
On the other hand, in case the film of the material, which is a member to be processed, is thin, interference fringes will not appear cyclically. In this case, therefore, an absolute film thickness can be found using an interference light that has two wavelengths.
According to the present invention, therefore, it is possible to provide a film thickness measuring method of members to be processed. The method can measure an actual thickness of a layer to be processed online precisely in plasma processing, especially in plasma etching, as well as a processing method of sample members to be processed with use of the measuring method.
Furthermore, it is possible to provide etching processing method that can control each layer of a semiconductor device to a predetermined thickness online precisely. It is also possible to provide a film thickness measuring apparatus of members to be processed. The apparatus can measure an actual thickness of a layer to be processed online precisely.
Furthermore, according to the present invention, it is possible to provide a film thickness measuring method of members to be processed. The method can measure an actual thickness of a layer to be processed online precisely in plasma processing, especially in plasma etching, as well as a processing method of sample members to be processed with use of the measuring method.
Furthermore, it is possible to provide an etching method that can control each layer of a semiconductor device to a predetermined thickness online precisely. It is also possible to provide a film thickness measuring apparatus of members to be processed. The apparatus can measure an actual thickness of such a layer to be processed online precisely.